1. Field of the Invention
The present invention relates to a semiconductor device having a heterostructure with a plurality of active material well layers alternately grown between a plurality of barrier layers and having a reduced net strain. More specifically, the present invention relates to an integrated semiconductor laser-modulator device exhibiting a reduced net strain, and to a process for manufacturing such a semiconductor device.
2. Description of the Related Art
During the manufacturing of heterojunction semiconductor devices, each individual layer is grown epitaxially using one of many known growth techniques such, for example, as metal-organic chemical vapor deposition (MOCVD). Several layers are usually grown when producing a multiple quantum well (MQW) semiconductor such as an MQW laser so that alternating layers of active material well layers and barrier material layers are grown. In addition, the alternating layers are bound between a pair of separate confinement layers (SCLs).
With respect to MQW semiconductor laser devices, it is known that a lattice mismatch between the alternating layers produces a strain in the active layer and affects the energy band structure of the quantum well and the wavelength of the light output from the quantum well. Furthermore, in line with continuing efforts to miniaturize and maximize the efficiency of semiconductor components, MQW semiconductor laser devices are being integrally designed with other element devices such as modulators. Referring to FIG. 1, a known integrated laser-modulator device 102 includes active layers 112 and barrier layers 110 comprising a plurality of quaternary layers bound between SCLs 118 grown on substrate 114. The laser region 106 of this device is grown by a selective area growth (SAG) process in which the epitaxial growth of the layers is confined between two pads 116 comprising, for example, an oxide. The laser region 106 and the modulator region 104 of the device are grown simultaneously. The growth of the laser region 106 between the two oxide pads causes an increase in the growth rate (thickness) and strain in the laser region 106 as compared to the planar growth in the modulator region 104. The resulting difference in thickness and strain causes the characteristic wavelength of the SAG or laser region to be longer than the characteristic wavelength of the modulator region.
It is also known that for satisfactory operation of an Electro-Modulated Laser or Electroabsorption-Modulated Laser (EML), which includes an integrated laser and modulator, a large change in strain is required in the well layer between the laser region and the modulator region of the device to effect the longer characteristic wavelength. However, the barrier and SCL layers do not require a large change in strain between the laser and modulator portions of the semiconductor device. It is also known that it is important to minimize the total amount of strain in the laser section. Each laser has a strain limit which is the maximum net strain that it can accommodate. The efficiency of a laser decreases as the net strain increases, so that the further the net strain is from the strain limit, the better. Designers must thus balance the requirement for a large change in strain between the laser and modulator with the relationship between the net strain and efficiency of the laser.
One prior art solution for affecting the strain is to carefully optimize the quaternary compositions used to form the semiconductor layers to achieve the desired characteristics of each layer. This procedure is however difficult to carry out because it generally requires time-consuming trial and error experimentation, yet yields only minimal improvements.